Most materials expand when heated as a consequence of the enhanced magnitude of thermal vibrations. See Robert Cahn's "Thermal Contraction of Oxides and Metals," Nature, 386:22-23 (March, 1997). Although unusual, certain materials are known that contract when heated. These materials also have thermal vibrations with enhanced magnitudes as a consequence of heating, and contraction on heating therefore must occur by some compensating mechanism or mechanisms. Some materials, such as the tungstate compounds, have bond vibrations that are postulated to cause concomitant shortening of other distances, which results in low or negative thermal expansion (NTE) properties. Additional discussion concerning proposed mechanisms for negative thermal expansion is provided by (1) Evans et al., Chem. Mater., 8:2809-2823 (1996), and (2) Sleight, Endeavor, 19:64-68 (1995).
Materials having very low thermal expansion are useful primarily because of their resistance to damage from thermal shock on rapid heating or rapid cooling. There are applications for negative thermal expansion materials in a pure form. However, the primary applications for such materials is to adjust to lower values the thermal expansion of metal-, oxide- or polymer-based compositions. Some negative thermal expansion materials are known which expand or contract isotropically. "Isotropic" refers to equal expansion or contraction in all dimensions. Examples of negative thermal expansion materials that expand and contract isotropically include (1) ZrVPO.sub.7, HfVPO.sub.7 and related compounds, discussed in Sleight's U.S. Pat. Nos. 5,322,559 and 5,433,778, and (2) ZrW.sub.2 O.sub.8 and HfW.sub.2 O.sub.8, discussed in Sleight et al.'s U.S. Pat. No. 5,514,360. U.S. Pat. Nos. 5,322,559, 5,433,778 and 5,514,360 are incorporated herein by reference. Both cubic and amorphous solids must provide isotropic thermal expansion. Most of the known isotropic negative thermal expansion materials have cubic crystal structures, and the symmetry of the cubic structures forces materials to expand and contract equally in all dimensions.
Anisotropic expansion means that crystallites of a compound expand in certain dimensions while contracting in at least one dimension. The magnitude of the contraction (negative expansion) in a first direction may be offset by expansion in a second direction. Hence, even though the sum of the expansion in all dimensions (the bulk expansion) may be negative, the magnitude of the negative expansion is reduced. Although isotropic materials generally are preferred, anisotropic materials also are useful. But, a material can expand or contract too anisotropically to be useful for a particular application. For example, anisotropic expansion or contraction can cause cracking in brittle materials, such as ceramics, especially when used in pure form rather than as composites.
In addition to Sleight's negative thermal expansion materials mentioned above, other low or negative thermal expansion materials are known. For example, .beta.-eucryptite exhibits a very small volume thermal expansion. Apparently, .beta.-eucryptite expands in one direction of the material's unit cell (a unit cell is defined as the simplest, three dimensional polyhedron that by indefinite repetition makes up the lattice of a crystal and embodies all the characteristics of its structure), and contracts in a second direction. "Thermal Contraction of .beta.-Eucryptite (Li.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2) by X-ray and Dilatometer Methods," J. Am. Ceram. Soc., 42:175-177 (1959). The overall thermal expansion of .beta.-eucryptite is reported to be either slightly positive or slightly negative. Zirconyl phosphate (ZrO).sub.2 P.sub.2 O.sub.7 ! is a largely anisotropic material that expands in two directions while actually contracting in only one direction. More specifically, the a and c axes of the unit cell for (ZrO).sub.2 P.sub.2 O.sub.7 expand continuously with increasing temperature, while the b axis contracts. The net effect is a small volume contraction over a limited temperature range. "Low-Thermal-Expansion Polycrystalline Zirconyl Phosphate Ceramic," J. Am. Ceram. Soc., 68:273-278 (1985). Scandium tungstate Sc.sub.2 (WO.sub.4).sub.3 ! apparently has negative thermal coefficients in two dimensions and a positive thermal coefficient in a third dimension. V. A. Balashov et al's "Growth and Certain Properties of Sc.sub.2 (WO.sub.4).sub.3 Crystals," translated from Izvestiya Akademii Nauk SSSR, Neorganicheskie Matenaly, 11(9): 1712-1714 (1975). There is some disagreement, however, concerning the magnitude and directions of the thermal coefficients for scandium tungstate. Moreover, the thermal expansion of scandium tungstate is highly anisotropic.
Most known compounds that exhibit NTE do so only well below room temperature or well above room temperature. Low temperature examples include Cu.sub.2 O, Si and amorphous SiO.sub.2. High-temperature examples include ZrV.sub.2 O.sub.7 and crystalline forms of SiO.sub.2. Such materials are not suitable for most applications because useful temperatures for most applications generally are about room temperature. Compounds which do show NTE at room temperature (e.g., .beta.-eucryptite) are highly anisotropic. There is thus a need for new materials which show NTE at room temperature, particularly if they are isotropic or nearly so. There also is a need for a family of compounds whereby thermal expansion can be varied from slightly positive, to zero, to strongly negative. Such a family also is needed so that other important properties, such as density, refractive index and dielectric properties can be varied and optimized for particular applications.